1. Field of the Invention
The present invention relates to a sheet postprocessing apparatus which applies postprocessing such as a punching process, binding process, and folding process to a sheet (to be also referred to as a recording sheet or transfer sheet) discharged from an image forming apparatus such as an electrophotographic copying machine, a printer, a facsimile apparatus, or a composite apparatus having functions equivalent to them after an image is transferred/formed on the sheet by the image forming apparatus.
2. Description of the Prior Art
There has been provided a sheet postprocessing apparatus which performs postprocessing such as a punching process, folding process, and binding process for a sheet onto which an image is transferred by an image forming apparatus such as a copying machine, a printer, a facsimile apparatus, or a composite apparatus of them. This sheet postprocessing apparatus is driven upon being connected to the print function of an image forming apparatus.
In the sheet postprocessing apparatus disclosed in Japanese Unexamined Patent Publication No. 2001-72321, the sheet bundle center folding section for center-folding a plurality of sheets executes a Z-fold process, internal three-fold process, center folding process, or the like for one sheet.
In the sheet postprocessing apparatus disclosed in Japanese Unexamined Patent Publication No. 2001-261220, the first postprocessing section executes a Z-fold process for one sheet, and the second postprocessing section located downstream of the binding section executes a center folding process for a plurality of sheets, and an internal three-fold process, a center folding process, or the like for one sheet.
As the conventional postprocessing apparatuses, there is a postprocessing apparatus, as shown in FIG. 1, in which some kinds of folding processes such as, for example, a center folding, three-fold, Z-fold, etc., are applied on the transfer sheet on which an image is formed by an image forming apparatus.
FIG. 1 is a longitudinal sectional view showing the arrangement of the main part of a conventional sheet postprocessing apparatus. The sheet postprocessing apparatus 60 is connected to an image forming apparatus, not shown, by disposing the sheet postprocessing apparatus so as to facing an inlet guide plate 621 for introducing a transfer sheet S into an inlet 62 of the sheet postprocessing apparatus to a discharg roller 61 for discharging the transfer sheet S on which an image is formed by the image forming apparatus.
In the vicinity of the inlet 62, a sensor PS1 for detecting leading and trailing ends of the transfer sheet S and a punching means 63 are arranged. In the downstream of the punching means 63, a resist roller 64 for correcting a position of the transfer sheet S. In a further forward portion thereof, a switching member 69 for switching a convey path of the transfer sheet S is so arranged as to be swingable by the action of a solenoid SD1.
On the upper side of the switching member 69, folding rollers 651, 652 and 653 are mounted such that pairs of folding rollers 651, 652 and 652, 653 abuts with each other under pressure, respectively. On the upper side of the folding rollers 651, 652 and 653, a first stopper means 66 is arranged. The first stopper means 66 consists of a pair of rollers and an endless belt wound over these rollers, and is driven by a motor M1 connected to one of the pair of rollers. To the endless belt, a collision member 661 is fixed so as not to move forwardly the transfer sheet S over the collision member 661.
On the lower side of the folding roller 653, there is provided a second stopper means 67 consisting of a pair of rollers, which have different diameters from each other, and an endless belt wound over the pair of rollers, and driven by a motor M2 connected to one of these rollers having a small diameter. The endless belt of the second stopper means 67 has a collision member 671 also. On the downstream side of the second stopper means 67, there is a provided a discharge roller 68 for discharging the transfer sheet S outside the sheet postprocessing apparatus 60.
In FIGS. 2A to 2C, operation steps for applying a Z-fold process by making use of the sheet postprocessing apparatus 60 are shown in order. In FIG. 2A, the transfer sheet S transferred from the image forming apparatus, not shown, is further transferred toward the first stopper means 66 by the action of the switching member 69 after correcting its position in cooperation with the resist roller 64. The leading end of the transfer sheet S is stopped by colliding with the collision member 61, but the resist roller 64 continuously rotates so as to further transfer the transfer sheet S. As a result, the transfer sheet S itself causes a first deflection in the vicinity of the folding rollers 651, 652. Further, when the trailing end portion of the transfer sheet S is continuously transferred, the transfer sheet S is rolled in a nip point N4 between the folding rollers 651 and 652 so as to form a fold e.
By rotating continuously the fold rollers 651, 652 in the direction of the arrows in FIG. 2A, the fold e is stopped by colliding with the collision member 671 of the second stopper means 67, as shown in FIG. 2B. Then, the transfer sheet S itself causes a second deflection when the fold rollers 651, 652 continue rotating. As a result, the transfer sheet S is rolled in a nip point N5 between the folding rollers 652 and 653 so as to form a fold f. Thus, the Z-fold process is completed.
In FIG. 2C, the transfer sheet S on which the fold f is formed is transferred again to the second stopper means 67 after turning around the periphery of the folding roller 653. At that time, the collision member 671 is moved by the action of the motor M2 shown in FIG. 1 to a position where a convey path toward the discharge roller 68 is opened. Therefore, the transfer sheet S on which the Z-fold process is applied can be discharged outside by the discharge roller 68.
Since the collision members 661, 671 can be freely displaced by the endless belt, it becomes possible to apply other folding processes except the Z-fold process such as, for example, a three-fold process, a center folding process, etc., to the transfer sheet S.
However, in the above-described conventional postprocessing apparatus 60, there is a problem such that a folding position is not stable because, dependent on the size of the transfer sheet S, the distance between the collision member 661 and the nip point N4 shown in FIG. 2A and the distance between the nip point N5 shown in FIG. 2B and the collision member 671 becomes long.
In order to overcome this problem, such an improved folding section as having two pairs of folding rollers and trailing rollers each abutted against the folding roller under pressure in which the two folding rollers are abutted against each other under pressure has been invented. According to the folding section, it becomes possible to attain a precise folding position and a stable folding process.
In the improved conventional postprocessing apparatus, two improved folding sections are arranged at a first folding section where a first folding process is applied to the transfer sheet S and at a second folding section where a second folding process is applied thereto. In case of applying the Z-fold process, the first folding process is applied to a predetermined position of the transfer sheet S at the first folding section, and then the second folding process is applied to the transfer sheet S, to which the first folding process has been applied, at the second folding section.
In FIGS. 3A and 3B, operation steps of second folding process in the Z-fold process by making use of the second folding section of the improved sheet postprocessing apparatus are shown in order.
The transfer sheet S is transferred from the first folding section, not shown, in the direction of the arrow T under the condition that a fold back portion of the transfer sheet S faces the folding rollers 91 and 92, and then stopped at a position corresponding to a half of the whole original length of the transfer sheet S by the cooperation of a sensor, not shown. As shown in the FIG. 3A, the transfer sheet S is subjected to buckling when the pairs of the folding rollers 91, 92 and the trailing rollers 93, 94 abutted against the folding rollers, respectively, are allowed to rotate in the directions of arrows, and advanced toward the nip point n.
At that time, as shown in FIG. 3B, only the leading end t of the fold back portion S′ folded in the first folding section firstly passes through the nip point n, and thereafter a deflection portion b of the transfer sheet S is rolled in the nip point n. As a result, when the second folding process is continued under this condition, the transfer sheet S is folded in the state that the leading end t is remarkably shifted from the fold b. Accordingly, such a phenomenon as called multiple folding is generated, so that the folding position is not stable.
The following problems are posed in these conventional sheet postprocessing apparatuses for performing folding processes.
(1) When the sheet bundle center folding section for performing a center folding process is to execute a folding process such as a Z-fold process, internal three-fold process, or center folding process for one sheet after a plurality of sheets are bound, excessive pressing force produced by a pair of folding rollers in tight contact with each other may be exerted on one sheet to produce creases on the sheet or damage it. When a folding plate is inserted between the pair of folding rollers in tight contact with a sheet being clamped between the rollers, the folding plate may damage the fold of the sheet. In addition, since a folding process such as an internal three-fold process or center folding process for one sheet is executed after the sheet is conveyed to the sheet bundle center folding section located downstream of the binding section, a sheet convey failure tends to occur on a long, bent sheet convey path.
(2) Consider a case wherein the first postprocessing section executes only a Z-fold process, and the second postprocessing section executes a center folding process for a plurality of sheets and a folding process such as an internal three-fold process or a center folding process for one sheet. In this case as well, when a folding process such as an internal three-fold process or center folding process is to be performed for one sheet, excessive pressing force produced by the pair of folding rollers in tight contact with each other may be exerted on one sheet to produce creases on the sheet or damage it. In addition, when the folding plate is inserted between the pair of folding rollers in tight contact with each other with a sheet being clamped between the rollers, the folding plate may damage the fold of the sheet. Furthermore, since a folding process such as an internal three-fold process or center folding process for one sheet is executed after the sheet is conveyed to the sheet bundle center folding section located downstream of the binding section, a sheet convey failure tends to occur on a long, bent sheet convey path.
(3) In the conventional sheet postprocessing apparatus, since sheets for which no folding process such as a Z-fold process, internal three-fold process, or center folding process is to be executed pass through the folding section, a sheet convey failure tends to occur in the folding section.